1. Field of the Invention
The present invention relates to a transmission line for transmitting a high-frequency signal of microwaves and millimeter waves, and to a transmitter-receiver such as a radar device or a communication device including the transmission line.
2. Description of the Related Art
As a transmission line for transmitting a high-frequency signal, a slot line, which is disclosed in S. B. Cohn: Slot Line on a Dielectric Substrate, IEEE MTT-17, PP. 768-778, October 1969, has been known. The slot line is formed by providing a conductive layer on a first principal surface of a dielectric substrate and by providing a rectangular slot in the conductive layer. In this slot line, a high-frequency signal forms a mode having an electric field which is parallel with the width direction of the slot and a magnetic field which is parallel with the longitudinal direction of the slot, and travels in the longitudinal direction of the slot.
Also, another transmission line is disclosed in Japanese Unexamined Patent Application Publication No. 8-265007. In this transmission line, a conductive layer is provided on each of the first and second principal surfaces of a dielectric substrate, each conductive layer is provided with a slot extending in a rectangular shape along the traveling direction of a high-frequency signal, such that the slots face each other.
In the transmission line (slot line) according to S. B. Cohn: Slot Line on a Dielectric Substrate, IEEE MTT-17, PP. 768-778, October 1969, a high-frequency signal easily radiates through the slot and a current flow concentrates near both ends of the slot. Accordingly, transmission loss disadvantageously increases.
On the other hand, in the transmission line according to Japanese Unexamined Patent Application Publication No. 8-265007, a high-frequency signal locally travels inside the dielectric substrate and the vicinity thereof, and thus transmission loss can be reduced compared to the above-described slot line. However, when the two slots formed in the first and second principal surfaces of the dielectric substrate are displaced with respect to each other, a high-frequency signal radiates from the first and second principal surfaces of the dielectric substrate, which results in an increase in transmission loss.